The present invention relates to a cathode structure for an electron tube, and particularly relates to a rapid start cathode structure suitable for a color cathode ray tube.
FIG. 1 shows a prior art rapid start cathode structure disclosed in Japanese Utility Model Publication No. 23954/1972. Heaters 1 are inserted in a cathode sleeve 2 and an electron emitting layer 3 is provided on a top surface 4 of the cathode sleeve. Heater 1 includes a tungsten coiled filament 7 and a sintered alumina insulating layer 8 covering the filament. The outer surface of heater 1 is substantially surrounded by sleeve 2. At first, cathode sleeve 2 is formed a sleeve of rectangular cross section as shown in FIG. 2 and an inner space of the sleeve is so designed to have a clearance when heaters 1 are inserted in the sleeve. Then, in a press forming step, a forming apparatus 10 is moved in a direction indicated by an arrow 11 to produce the relation between the cathode sleeve and the heaters mentioned above.
In this cathode structure, the cathode sleeve overlaps and contacts with the outer surfaces of the heaters. Thermal and a mechanical coupling between the heaters and the cathode sleeves is thereby greatly increased. Drawbacks caused by a gap therebetween are eliminated and high heating efficiency is obtained. Therefore, this cathode structure is useful as a rapid start cathode structure. However, this cathode structure has additional fatal drawbacks as follows.
First, a reaction shown by W+Al.sub.2 O.sub.3 .fwdarw.W.sub.m O.sub.n +Al (wherein m and n are integral number) occurs between the tungsten filament and the alumina insulating layer. Oxygen contained in the resultant W.sub.m O.sub.n diffuses into the cathode sleeve and reacts with the electron emitting layer, resulting in so called gas dope phenomenon. As a result, the life of the cathode structure is extremely shortened.
Second, the neck of the color cathode ray tube is commonly narrowed in diameter. However this prior art cathode structure cannot be practically narrowed. In a color cathode ray tube having an in-line type electron gun the ability to narrow neck diameter is important.